Previous techniques for the measuring or monitoring of surfactants included: wet chemical techniques; surface tension measurements; and, measurement of foaming.
In a wet chemical technique, surfactants are typically measured using methylene blue (a cationic dye) and chloroform or benzene. Initially, the methylene blue is added to a solution containing surfactants. A methylene blue/surfactant ion pair is then extracted into the organic phase and measured by absorption spectroscopy. The disadvantages of this method are the generation of hazardous wastes and the difficulty in adapting this technique to a continuous, on-line method.
Surfactants can also be detected by measuring the surface tension of a solution. Several methods can be used, e.g., a tensiometer can measure the surface tension by measuring the resistance to withdrawing a plate or ring from a solution. Also, a pulsating bubble surfactometer has been described and is particularly useful for screening lung surfactants. This device works by measuring the pressure difference across a pulsating bubble interface. Still further, the rate of drops forming and falling off a capillary tube has also been demonstrated as a method for measuring surface tension. While all these methods can provide an accurate measure of surface tension, none can be readily adapted to on-line measurements. Each of the above devices require a stable, vibration free, location to operate and all are more costly than the subject matter of the present invention.
Direct measurement of foaming has also been used to detect surfactants. Among previous devices or techniques for measuring foaming are the following. In a first technique known as the Ross-Miles test (ASTM D1173-53), the solution to be tested drips into a long tube. The height of the foam generated is a measure of the surfactant concentration. This method cannot be used on-line or for continuous measurement. This method has the additional drawback of requiring an operator present during the entire test.
A second technique (German Patent No. 3625817 A1 to Bonsels) involves a device to measure surfactants in washing machines. In this device, a solution is continuously pumped through a chamber and out an overflow. A foam is generated by introducing air under the chamber. The presence of foam is detected by a light beam above the surface of the solution. After the measurement is made a burst of air clears the foam out of the light path and the process begins again. Soap will build up on the windows as no provision for cleaning them is provided. This method gives no quantitative information about the surfactant present. It is not possible to adapt this device to quantitative analysis of the surfactant present. The method only determines if free surfactant is present above a predetermined level. No adjustment for the height of the sensor is used minimizing the flexibility of the instrument.
The third technique, U.S. Pat. No. 1,866,296 by H. J. Christmann, involves bubbling air through a column of liquid for a fixed length of time and measuring the foam and its rate of collapse. Like the Ross-Miles test this method cannot be used for continuous or on-line measurements and requires an operator.